In general, power quality problems in the power grid are mainly caused by various disturbances in the power grid. It covers a wide range of things, both short-term and short-term, as well as long-term or long-term: both amplitude and frequency issues. However, on the whole, the power quality problems can be mainly divided into two major categories: steady state and unsteady state.
With the in-depth research on the power quality of the power grid, various analysis methods based on digital technology are studying the propagation of harmonics in the power grid, analyzing the waveform distortion caused by various disturbance sources, and developing various power quality control devices and analyzing them. It has been widely used in applications such as the application of power quality problems.
If systematically analyzed, overall, the various analysis methods based on digital technology can be summarized as three types: time domain, frequency domain, and transform domain.
(1) Time Domain Simulation Method
The time domain simulation method is a method that uses various time domain computer simulation programs to study various transient phenomena that affect the power quality of the power grid. It is most widely used in power quality analysis. At present, the more common software that can perform time domain simulation are EMPT, EMPDC, NETOMAC and other system transient simulation programs and SPICE, PSPICE, ORCAD and MULTISIM and SABER and other electronic circuits and power electronic circuit simulation programs.
For the time domain simulation method of power quality, it mainly solves the problem of establishing a corresponding power system model, and carries out simulation analysis on the actual signal data of the collected power grid. The main disadvantage of the time-domain simulation method is that the maximum frequency range that they can emulate is limited by the simulation step size. Therefore, it is necessary to know the frequency coverage of the transient process in advance, and in some simulation situations, it may cause numerical oscillations. Causes inaccurate results or inability to get correct results.
(2) Frequency Domain Analysis Method
The frequency domain analysis method is mainly used to analyze the harmonic issues affecting the power quality of the power grid. It includes frequency sweeps and harmonic current calculations.
The frequency domain analysis method of power quality mainly obtains the required result by solving the equation reflecting the relationship between the injected current vector and the node voltage vector in the corresponding network. The establishment of the equation is the key to the successful implementation of the method. When the corresponding current components are taken from each nonlinear load current to form the node injection current vector, the magnitude of these nonlinear load model errors has a great influence on the accuracy of the analysis results.
The main disadvantage of the frequency domain analysis method is that the establishment of corresponding equations is cumbersome and complicated, and the calculation amount is also large.
(3) Transform-based methods
The transform-based methods mainly refer to the Fourier transform (FT) method, the short-time Fourier transform (STFT) method, the quadratic transform (QT) method, and the wavelet transform (WT) method.
Fourier transform is a pure frequency domain analysis method. Its localization in the frequency domain is completely accurate, but it has no localization in the time domain.
In view of the deficiencies of the Fourier transform, in order to study the frequency domain characteristics of the signal in the local time range, scholars have proposed many new improvements, one of which is the famous Gabor transform proposed by Gabor in 1946 and the short term that has been developed later. Fourier transform (ST FT). However, since the definition of the STFT determines that the size and shape of its window function are constant regardless of time and frequency, that is, this window size is fixed for all time periods and frequency bands of the analyzed signal. Therefore, it is only suitable for the analysis of the process with roughly the same feature size, and is not suitable for the analysis of multi-scale processes and mutation processes. Therefore, the use of STFT cannot accurately analyze the time-varying signals, of course, it cannot achieve accurate positioning of the signal mutation points. .
The second transform is a time-frequency transform method based on the energy angle [7]. It is based on the bilinear function that the signal energy distribution is time and frequency and thus can constitute a quadratic transformation. The earliest and most common time-frequency energy density is the Winger-Ville distribution. Although the Winger-Ville distribution has some good properties, its application is limited by the presence of distractors. It is of course possible to use time-frequency averaging to reduce these interferences, but at the expense of some time-frequency resolution [8].
Wavelet transform is also a time-frequency domain analysis method. Its greatest feature is that it can automatically adjust the size of the time-frequency domain window based on the difference in the analyzed signal frequency, and observe and analyze the characteristics of the signal with the required accuracy from the time-frequency domain. In particular, the wavelet transform can also be rapidly implemented through an efficient algorithm, making it possible to monitor the power quality of the power grid in real time. As its good characteristics are continuously revealed in the research of power quality of power grids, wavelet transform is becoming a powerful tool for power quality analysis, and it has been widely used in the solution of practical problems and has achieved some value. The results.
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